1. Field of the Invention
Exemplary embodiments of the present invention generally relate to a developing unit, an image forming apparatus including the developing unit, and a method of controlling amounts of toner used in the image forming apparatus, and more particularly, to a developing unit that employs a two-component developer including toner particles and carrier particles, an image forming apparatus including the developing unit, and a method of controlling amounts of toner used in the image forming apparatus incorporating the developing unit.
2. Discussion of the Related Art
Developing units that develop toner images for electrophotographic printing generally employ either a one-component developer or a two-component developer. While the one-component developer includes toner particles only, the two-component developer includes toner particles and magnetic carrier particles to which the toner particles adhere. The two-component developer is widely used in developing units where the two-component developer is mixed in a developer container so as to frictionally charge the two-component developer (hereinafter “developer”) so that a developer bearing member holds the charged developer thereon. Toner particles or toner in the developer carried by the developer bearing member then selectively adhere to an electrostatic latent image so that a visible toner image can developed thereat.
FIG. 1 illustrates a schematic configuration of a dual-axis type of related-art developing unit 1000 employing a two-component dry developer in which the developer unit 1000 is viewed end-on, that is, with its axis perpendicular to the sheet of paper on which the drawing appears.
As shown in FIG. 1, the related-art developing unit 1000 that is generally incorporated in an image forming apparatus includes a developing roller 1001, a first path 1002, a second path 1003, a first conveyance screw 1004, and a second conveyance screw 1005.
The first conveyance screw 1004 and the second conveyance screw 1005 are provided in the first path 1002 and the second path 1003 in a long axial direction of the developing roller 1001 that serves as a developer bearing member. Developer travels in opposite directions in the first path 1002 and the second path 1003, propelled by the first conveyance screw 1004 and the second conveyance screw 1005 to the developing roller 1001 while being agitated. The dual-axis type developing unit 1000 collects any remaining developer and recirculates it.
When such an image forming apparatus continues to output images with a low image area ratio (i.e., images in which the actually printed area is small relative to the total surface area of the recording medium on which the image is formed), an amount of toner consumed is also relatively small while the toner remaining in the developing unit 1000 continues to be agitated by the conveyance screws. Such continuous agitation can subject the developer to excessive mechanical stress that degrades both toner and carrier. Specifically, such stress may, for example, cause additives to the toner to be embedded in the toner particles or the toner to be charged up or excessively charged. Deterioration of the toner and carrier in turn can cause such problems as a decrease in transfer efficiency of the toner from carrier to member/roller to final medium and a decrease in the amount of toner attracted to the developing roller, leading to inconsistent image density for output images.
By contrast, when images with a high image area ratio are continuously output, new toner, which corresponds to toner remaining in the developing unit 1000 until being consumed for image developing, may be conveyed to a development area without being agitated sufficiently. In this case, the toner receives less mechanical stress than the above-described case of continuously outputting images with a low image area ratio. However, the amount of charged toner tends to decrease, and it is likely that an insufficient amount of charged toner affects image density and/or color. If the above-described state continues, problems such as toner scattering and image background contamination may occur. In short, when toner agitation time/degree shifts or deviates significantly from a suitable range depending on the image area ratio, various problems may occur.
To eliminate the above-described drawbacks, one proposed approach suggests an image forming apparatus including a toner discharging mode. When images with a low image area ratio are continuously output, degraded toner particles are discharged by force and replaced with new toner particles.
Therefore, even when images with a low image area ratio are continuously output, the output images maintain a given optimum image density. However, the toner yield or productivity is degraded, causing an increase in running costs and posing a large problem for the environment.
Another proposed approach provides an image forming apparatus in which the charged-up toner and carrier are separated. Specifically, a video counter incorporated in the image forming apparatus counts an image area ratio, and when images with a low image area ratio are continuously output, compressed air is blown onto the surface of the developing roller to separate the charged-up toner and carrier for reuse of the separated toner.
According to the image forming apparatus with this technique, the separation of the charged-up toner and carrier can reduce the amount of charged toner. However, such an image forming apparatus may need to incorporate additional components such as a video counter, an air compressor, and a toner vacuum, resulting in an increase in size of the image forming apparatus, which runs counter to increasing market demand for more compact image forming apparatus and causes an increase in costs.
In sum, it is contemplated that problems of an image forming apparatus incorporating a developing unit with a two-component developer system occur for the following reasons: When a related-art image forming apparatus continuously outputs images with a low image area ratio, the toner contained in the developing unit receives a large amount of stress but most of the toner is left unconsumed for development. On the other hand, when the related-art image forming apparatus continuously outputs images with a high image area ratio, a large amount of new toner is replaced, and therefore most of the toner contained in the developing unit receives little stress but is consumed for development while insufficiently charged.
Accordingly, the following two possible countermeasures to decrease mechanical or agitation stress per toner particle suggest themselves:
(1) Increasing the amount of developer in the developing unit or increasing the capacity of the developing unit; and
(2) Increasing the toner density.
However, (1) cannot avoid an increase in size of the image forming apparatus, and thus is not suitable for image forming apparatuses with a space limitation. On the other hand, when a toner density with a carrier coverage over 100% is set as in (2), some toner cannot contact the carrier and thus cannot be charged. Therefore, in principle (2) is not an option in an electrophotographic developing unit in which electrostatic force is used. Further, no large toner increase can be expected within the above-described limitations, and therefore it is difficult to expect any great decrease in toner deterioration.
Further, as noted above, a developing unit of the dual-axis type shown in FIG. 1 employs a configuration in which developer that has been supplied to a developer bearing member is returned to a supply/conveyance path of the developer. That is, developer that has passed a development area in which a certain amount of toner on the surface of the developer bearing member has been consumed to develop an image and developer that has yet to be supplied to the surface of the developer bearing member are mixed in the supply/conveyance path. Therefore, the amount of developer that has passed the development area may be greater as the developer is conveyed further downstream in a direction of conveyance of the supply/conveyance path and the toner is consumed in the process of forming an image. In other words, the toner density of the developer supplied to the developer bearing member may gradually decrease as the developer is conveyed further downstream in the direction of conveyance of the supply/conveyance path. Accordingly, a difference in output toner densities arises between upstream side and downstream sides in the direction of supply and conveyance of the developer, and this difference can adversely affect image quality.
To eliminate the above-described drawbacks, providing an image forming apparatus including a developing unit with a collection/conveyance path has been proposed. That is, separate from the supply/conveyance path to supply developer to a developer bearing member, the developing unit includes a collection/conveyance path where the developer that has passed the development area is collected to a new toner inlet that is a starting portion of the supply/conveyance path. According to the image forming apparatus including this approach, inconsistency in the densities of output images can be reduced. However, this approach cannot avoid the toner deterioration such as the above-described mechanical stress due to agitation and toner charge-up.